1. Field of the Invention
The present invention relates to a nonaqueous electrolyte and a nonaqueous electrolyte secondary battery.
2. Description of the Related Art
At present, as nonaqueous electrolyte secondary batteries for use in portable devices such as portable telephones or the like, lithium ion secondary batteries are commercialized. As an example of the lithium ion secondary battery, one that includes a positive electrode containing a lithium cobalt composite oxide (for instance, LiCoO2), a negative electrode containing a graphitized material or a carbonaceous material, a liquid nonaqueous electrolyte containing an organic solvent therein a lithium salt is dissolved, and a porous film that is a separator is known. As the organic solvent a nonaqueous solvent of low viscosity and low boiling point is used.
For instance, in Jpn. Pat. Appln. KOKAI Publication No. 4-14769, a nonaqueous electrolyte secondary battery comprising a liquid nonaqueous electrolyte that is mainly made of a solvent mixture in which propylene carbonate, ethylene carbonate and γ-butyrolactone are mixed, and has a ratio of γ-butyrolactone in the range of 10 to 50% by volume with respect to a total solvent mixture is described. In the publication, γ-butyrolactone is added to a solvent mixture of propylene carbonate and ethylene carbonate to increase the conductivity of the liquid nonaqueous electrolyte at low temperatures, and thereby low temperature discharge characteristics of a cylindrical nonaqueous electrolyte secondary battery are improved.
However, in the nonaqueous electrolyte secondary battery disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-14769, a lot of gas is generated from a negative electrode at the initial charge, or when stored at extremely high temperatures of 60° C. or more, the positive electrode reacts with the liquid nonaqueous electrolyte and an oxidative degradation of the liquid nonaqueous electrolyte is caused, and thereby a gas is generated. Accordingly, when in order to make a thickness of a battery thinner, a wall thickness of a case that accommodates the positive electrode, the negative electrode, the separator and the liquid nonaqueous electrolyte is made thinner, there occurs a problem in that the case swells owing to the generated gas, resulting in deforming. When the case is deformed, the battery may not be fitted into an electronic device, or malfunction of the electronic device may be caused. Furthermore, in the nonaqueous electrolyte secondary battery, there is a problem in that under a high temperature environment, a reaction between the negative electrode and γ-butyrolactone tends to proceed, accordingly a longer life in the charge-discharge cycle at high temperatures cannot be obtained.